BRUSSELS (AP) — The leaders of Serbia and Kosovo negotiated Tuesday on one of the most important issues dividing them, as Serbia strains to meet conditions for eventual membership in the European Union.

Talks between the prime ministers of the two countries have reached a “decisive stage” as the two focus on the thorny issue of the status of Serb-dominated northern Kosovo, an EU official said. The official spoke on condition of anonymity in order to discuss closed-door talks.

The talks — the eighth face-to-face session between Serbian Prime Minister Ivica Dacic and Kosovo Prime Minister Hashim Thaci — are being held under the mediation of EU foreign policy chief Catherine Ashton.

“Today is a historical day,” said Thaci as he entered Ashton’s office building Tuesday morning to begin the talks. “It is a decisive moment for Kosovo and for the region.”

In early afternoon, a spokeswoman for Ashton said that bilateral talks between Ashton and each of the two prime ministers had concluded and the three were now meeting together.

Kosovo, a former Serbian province, declared independence in 2008. While many countries have recognized it as an independent country, Serbia has not.

The ethnic Serbs living in northern Kosovo — up to 50,000 people in and around the divided city of Mitrovica — have rejected the authority of the government in Pristina, the Kosovo capital. They have created so-called parallel institutions, including hospitals and schools, all financed and supported from the Serbian capital, Belgrade.

However, in order for Serbia to join the EU — something it dearly wants to do — it must normalize relations with Kosovo. That includes not only resolving such ordinary issues as trade and border control, but also resolving the status of northern Kosovo — and whether it does or does not fall under the authority of the government in Pristina.

Details of any possible deal remain murky, and its implementation would have many question marks, said Agron Bajrami, editor in chief of the largest Kosovo daily, Koha Ditore.

“Everybody is waiting to see what will it look like, but the greatest fear, the greatest risk, is that even if there is a deal we don’t know how this deal will be implemented,” Bajrami said. “There might be complications, especially in the north where the deal will have to address several very difficult issues — security, political and social, as well.”

Serbs in Kosovo’s north say that any agreement that separates them from Serbia would not be acceptable.

“No way will we go with the Pristina authorities,” said Tomislav Kostic, a resident of Mitrovica. “Only with the state of Serbia and that’s it.”

In the southern part of Mitrovica, ethnic Albanian Adem Mripa said the territory was part of Kosovo. He demanded reciprocity for the Albanian minority in Serbia’s south.

“If they want autonomy here they should give it to Albanians in the south, to the Hungarian and Bosnian (minorities), Mripa said. “I am for talks, but talks that are forward-looking and visionary and peaceful.”

A spokesman for the Kosovo government said the negotiations in Brussels could run on into Wednesday.

In a sign of the underlying tensions, Kosovo police said unknown assailants threw a fire bomb into the offices of moderate Serb leader Oliver Ivanovic in the Serb-run part of the country. No one was in the office when the attack was launched, minutes before midnight Monday. Ivanovic backs Serbia’s claim over Kosovo, but many radical Serbs think that he might work with the ethnic Albanian authorities in Pristina.

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Associated Press videojournalist Sylvain Plazy in Brussels, and AP writers Radul Radovanovic and Nebi Qena in Mitrovica, Kosovo, contributed to this report. Don Melvin can be reached at https://twitter.com/Don_Melvin